Step 2 : The index of the above pdf is actually the topics from which UPSC asks questions. So prepare basics of those topics from this book or any book like this. You can also have a look on the type of questions asked by UPSC on those topics.Google Drive Link Click Here

Step 3 : After you have gone through the questions you will realize that UPSC asks either very basic questions or questions related to current issues on above topics. Basic you can prepare from above pdf. Current issues can be prepared from this booklet.visionias

Step 4 : Earlier UPSC used to ask questions on Science from NCERTs. But in recent times i.e from 2011 onwards the number of questions from NCERTs has reduced. (You will come to know if you go through all past papers from 1979). You can get all Science NCERTs from hereGoogle Drive Link Click Here